An efficiency of a vehicle engine may be reduced during a cold start and/or prior to the engine reaching steady state operating temperatures. For example, combustion may be less complete when the engine is started compared with when the engine is warmed up, reducing fuel economy and increasing vehicle emissions. In particular, the engine may be run intermittently when the vehicle is a hybrid, equipped with a stop/start system, etc., which may result in multiple engine starts within a single drive cycle. Therefore, systems and methods that increase combustion efficiency during initial operation are of increasing importance. Factors that influence combustion completion include combustion chamber wall temperature, combustion air temperature, fuel temperature, and engine speed. In particular, heating the combustion air (e.g., air used in a combustion reaction within an engine cylinder) may increase combustion completion due to the influence of the combustion air temperature on mixture preparation and air charge reduction.
Other attempts to address air heating include operating in an air heating mode during a starting sequence of an engine. One example approach is shown by Clarke et al. in U.S. Pat. No. 5,117,790. Therein, one or more cylinders of a multi-cylinder engine are operated in the air heating mode prior to initiating combustion within the one or more cylinders by deactivating their exhaust valves. Once each of the one or more cylinders is sufficiently heated, the corresponding exhaust valves are activated and fuel is injected to initiate combustion. Additional cylinder(s) may then be operated in the air heating mode prior to initiating combustion in the additional cylinder(s).
However, the inventors herein have recognized potential issues with such systems. As one example, sequentially heating and initiating combustion in the cylinders of a multi-cylinder engine may prolong engine start times, resulting in decreased vehicle operator satisfaction. Furthermore, the engine may benefit from continued air heating even after the first combustion event, especially while the engine remains below steady state operating temperatures. Further still, hotter combustion air temperatures may aid certain ignition strategies, such as compression ignition strategies, even while the engine is warmed up.
In one example, the issues described above may be addressed by a method for a hybrid electric vehicle, comprising: during an engine start, deactivating engine cylinder exhaust valves while activating engine cylinder intake valves and electrically spinning the engine unfueled until reaching a threshold intake air temperature; and, after reaching the threshold intake air temperature, activating and fueling one or more of the cylinders to initiate combustion, and then alternating between deactivating and combusting in the one or more cylinders until reaching a threshold engine temperature. In this way, the engine may be efficiently started and with increased combustion completion.
As one example, alternating between deactivating and combusting in the one or more cylinders includes maintaining the exhaust valves of the one or more cylinders closed while the corresponding intake valves remain active and disabling fueling to the one or more cylinders over an engine cycle; and lifting the intake valve and the exhaust valve at corresponding valve timings, providing fuel via a fuel injector coupled to each of the one or more cylinders, and providing spark via a spark plug coupled to each of the one or more cylinders over the subsequent engine cycle. By alternately deactivating and combusting in the one or more cylinders, continued air heating may be provided even after the engine is started and combustion is initiated. For example, the threshold engine temperature may be a steady state operating temperature of the engine. Thus, continued air heating may be provided when the engine is colder. In contrast, air heating prior to a first combustion event may be performed independent of the engine temperature. In this way, the air heating prior to the first combustion event may be provided even when the engine has not substantially cooled, such as when the engine is shut down and restarted while the vehicle remains on. In still other examples, the continued air heating may be provided to facilitate operation in a compression ignition mode in which increased air charge temperatures may aid compression ignition. Overall, fuel economy may be increased and vehicle emissions may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.